[PATCH 0/3] perf: Fix libdw addr2line fallback, pr_debugN overhead, and cumulative hist decay

[Ian Rogers <irogers@google.com>]

Three standalone fixes found while profiling `perf top` and DSO symbol/DWARF
resolution:

1. **perf libdw: Optimize fallback logic for missing DWARF**:
   `libdw__addr2line()` previously returned `-1` when `dwfl_module_getdwarf()`
   or `dwfl_module_getsrc()` returned `NULL`, while `addr2line()` in
   `srcline.c` only stopped its fallback loop on `ret > 0`. This caused
   `perf` to fall back through `llvm`, `libbfd`, and the external
   `addr2line` subprocess on every stripped DSO or unmapped address, and
   kept the `Dwfl` ELF mmap open on stripped binaries. Standardize backend
   return values so `0` denotes conclusively missing DWARF/line info
   (stopping the fallback loop and freeing the `Dwfl` session when no
   DWARF exists), `< 0` denotes backend/initialization error, and `> 0`
   denotes resolved source line info.
   An earlier version of this patch was posted at:
   https://lore.kernel.org/linux-perf-users/20260824062841.1529489-3-irogers@google.com/
   This version addresses the review feedback from that thread.

2. **perf debug: Avoid calling eprintf() in pr_debugN() when verbose < n**:
   `pr_debugN()` unconditionally evaluated its arguments and called
   `eprintf()` -> `veprintf()` before checking `verbose >= n`, accounting
   for ~15% of total CPU time in `perf top` during ELF symbol loading even
   at default `verbose == 0`. Guard `eprintf()` with `unlikely(verbose >= (n))`.

3. **perf hist: Do not delete cumulative caller entries before stat_acc decays**:
   When `symbol_conf.cumulate_callchain` is enabled (`perf top -g`),
   callchain caller entries with `he->stat.period == 0` were deleted on the
   first `hists__decay_entries()` pass even while `he->stat_acc->period > 0`.
   Require `he->stat_acc->period == 0` before deleting entries in
   cumulative mode.

Assisted-by: Antigravity:gemini-3.1-pro

Ian Rogers (3):
  perf libdw: Optimize fallback logic for missing DWARF
  perf debug: Avoid calling eprintf() in pr_debugN() when verbose < n
  perf hist: Do not delete cumulative caller entries before stat_acc
    decays

 tools/perf/tests/pmu-events.c      |  5 ++-
 tools/perf/util/addr2line.c        | 28 ++++++++++---
 tools/perf/util/debug.h            |  5 +--
 tools/perf/util/dso.h              |  2 +-
 tools/perf/util/hist.c             |  6 ++-
 tools/perf/util/libbfd.c           |  6 +--
 tools/perf/util/libdw.c            | 25 ++++++++---
 tools/perf/util/libdw.h            |  5 ++-
 tools/perf/util/llvm-c-helpers.cpp | 66 +++++++++++++++++++-----------
 tools/perf/util/llvm.c             | 12 ++++--
 tools/perf/util/srcline.c          | 21 +++++++---
 11 files changed, 125 insertions(+), 56 deletions(-)

-- 
2.56.0.307.g5fe7bd680c-goog